Conversion of hydrocarbons



Patented May 9, 1939 I UNITED STATES convcasron or nrnnoomoNs Aristid V. Grosse and Jacque C. Morrell, i'iliicago,

Ill., assignors to Universal Oil Products Company, Chicago, Ill., a corporation of Delaware No Drawing. Application September 30, 1937,

Serial No. 166,584

4 Claims. '(Cl. 260-608) This invention relates particularly to the conversion of straight chain hydrocarbons into closed chain or cyclic hydrocarbons. v

More specifically it is concerned with a process involving the use of special catalysts and specific conditions of operation in regard to temperature, pressure and time of reaction whereby aliphatic hydrocarbons can be efficiently converted into aromatic hydrocarbons.

In the straight pyrolysis of pure hydrocarbon mixtures such as those encountered in fractions from petroleum or other naturally occurring or synthetically produced hydrocarbon mixtures the reactions involved which produce aromatics from. paraflins and olefins are of an exceedingly complicated character and cannot be very readily controlled.

It is generally recognized in the thermal decomposition of hydrocarbon compounds or hy-.

drocarbon mixtures or relatively narrow range that, whatever intermediate reactions are involved, there is an overall loss of hydrogen, a tendency to carbon separation and a generally wider boiling range in the total liquid products as compared with the original charge. Under mild cracking conditions involving relatively low temperatures and'pressures and short times of exposure to cracking conditions it is possible to some extent to control cracking reactions so that they are limited to primary decompositions and there is a minimum lossof hydrogenand a maximum production of low boiling fractions consisting of compounds representing the fragments of the original high molecular weight compounds.

As the conditions of pyrolysis are increased in severity using higher temperatures and higher times of exposure to pyrolysis conditions, there is a progressive increase in loss of hydrogen and a large amount of secondary reactions involving recombination of primary radicals to form polymers and some cyclization to form naphthenes.

and aromatics, but the mechanisms involved in these cases are of so complicated a nature that any particular success has attended the conversion of any given parafiin or other aliphatic hydrocarbon into an aromatic hydrocarbon of the same number of carbon atoms by way of the prois in a more or less empirical state.

' petroleum.

catalysts even in-connection with conversion reactions among pure hydrocarbons and particularly in connection with the conversion of the relatively heavy distillates and residue which are H available-for cracking, there is a general tendency for the decomposition reactions to proceed at a very rapid rate, necessitating the use of extremely short time factors and very accurate control of temperature and pressure to avoid too expensive decomposition. There are further difficulties encountered in maintaining the emciency of catalysts employed in pyrolysis since there is usually a rapid deposition of carbonaceous materials on their surfaces and in their pores.

The foregoing brief review of the art of hydrocarbon pyrolysis is, given to furnish a general background for indicating the improvement in such processes which is embodied in the present invention, which, may be applied to the treatment of pure paraflin or olefin hydrocarbons, hydrocarbon mixtures containing substantial percentages of paraffin hydrocarbons such as relatively close out fractions producible by distilling petroleum, and analogous fractions which contain un saturated as well as saturated straight chain hydrocarbons, such fractions resulting from cracking operations upon the heavier fractions of In one specific embodiment the present invention comprises the conversionof aliphatic hydrocarbons including parailin andolefln hydrocarbons into aromatic hydrocarbons by subjecting denum oxides, after which the remainingamount f of chromium oxides is deposited. This procedure may be modified so that molybdenum oxides are first deposited, then chromium oxides and lastly further amounts of molybdenum oxide or any amount or method of alternation. may be employed. The advantages in this method of prepa ration are definite although the reasonsv are not fully understood. v

or straight chain hydrocarbons having six or more carbon atoms in chain arrangement in their structure are catalytlcally dehydrogenated in such a way that the chain of carbon atoms undergoes ring closure with the production in the simplest case of benzene from n-hexane or n-hexene and in the case of higher molecular weight parafiins of various alkyl derivatives of benzene. of times of contact, temperature and pressure very high yields of the order of 75 to 90% of the benzene or aromatic compounds are obtainable which are far in excess of any previously obtained in the art either with or without catalysts. For the sake of illustrating and exemplifying the types of hydrocarbon conversion reactions which are specifically accelerated under the preferred conditions by the present types of catalysts, the following structural equations are introduced:

CH] CH In the foregoing table the structural formulas of the primary paraffin hydrocarbons have been represented as a nearly closed ring instead of by the usual linear arrangement for the sake of indicating the possible mechanisms involved. No attempt has been made to indicate the possible intermediate existence of mono-olefins, dioleflns, hexamethylenes or alkylated hexamethylenes which m ght result from the loss of various amounts of hydrogen. It is not known at the present time whether ring closure occurs at the loss of one hydrogen molecule or whether dehydrogenation of the chain carbons occurs so that the first ring compound formed is an arcmatic such as benzene or one of its derivatives.

The above three equations are of a relatively simple character indicating generally the type of reactions involved but in the case of n-paraffins or mono-olefins of higher molecular weight than the octane shown and in the case of branched chain compounds which contain various alkyl substituent'groups in dififerent positions along the six-carbon atom chain, more complicated reactions will be involved. For example, in the case of such a primary compound as 2,3 di-methyl hexane the principal resultant product is apparently o-xylene although there are concurrently produced definite yields of such compounds as ethyl benzene indicating an isomerization of two substituent methyl groups. In the case of nonanes which are represented by the compound 2,3,4-trimethyl; hexane, there is Under properly controlled conditions formation not only of mesitylene but also of such compounds as methyl ethyl benzol and various propyl benzols.

It will be seen from the foregoing that the scope of the present invention is preferably limited to the treatment of aliphatic hydrocarbons which contain at least 6 carbon atoms in straight chain arrangement. In the case of paraffin hydrocarbons containing less'than' 6 carbon atoms in linear arrangement, some formation of aromatics may take place due to primary isomerization reactions although obviously the extent of these will vary considerably with the type of compound and the conditions of operation. The process is readily applicable to paraffins'from hexane up to dodecane and their corresponding olefins. With increase in molecular weight beyond this point the percentage of undesirable I use of a particular group of composite catalytic materials which employ as their base catalysts or supporting materials certain refractory oxides and silicates which in themselves may have some slight specific catalytic ability in the dehydrogenation and cyclization reactions but which are improved greatly in this respect by the addition of chromium and molybdenum oxides in minor proportions. The base supporting materials are preferably of-a rugged and refractory character capable of withstanding the severe'use to which the catalysts are put in regard to temperature during service and in regeneration by means of air or other oxiding gas mixtures after they have become fouled with carbonaceous deposits after a period of service. As examples of materials which may be employed in granular, form as supports for the preferred catalytic substances may be mentioned the following:

Magnesium oxide, aluminum oxide; bauxite;

bentonite clays; montmorillonite clays; kieselguhr; crushed firebrick; crushed silica; glauconite (greensand) a It should be emphasized that in the field of catalysis there have been very few rules evolved which will enable the prediction of what materials will catalyze a given reaction. Most of the catalytic work has been done on a purely empirical basis, even though at times certain groups of elements or compounds have "been found to be more orless equivalent in accelerating certain types of reactions.

In regard to the ase catalytic materials which are employed as supports according to the present invention, some precautions are necessary to insure that they possess proper physical and chemical characteristics before they receive the depositions of chromium and molybdenum oxides.

In regard to magnesium oxide, which may be alternatively employed, this is most conveniently prepared by the calcination of the mineral mag-' reasonable figure. The pure compound begins pared by precipitation or other chemical methods may be used alternatively in place of the natural mineral. It is not necessary that the carbonate be completely converted to oxide but as a rule it is preferable-that the conversion be at least over 90%, that is, so that there is less than 10% of the carbonate remaining in the ignited material.

A very effective catalyst consists of activated aluminum oxide (preferably slightly alkalized) supporting successive depositions of about 2% chromium sesquioxide CI'2O3, 4% of molybdenum sesquioxide M020: and another 2% of the chroi'iium compound. The exact method of preparaon of this type of catalyst will be described in :tter paragraphs in greater detail. It is essenwal to the preparation of these catalysts that he aluminum oxide possess certain structural manufacture of catalysts for the-process may be obtained from some natural aluminum oxide minerals or ores such as bauxite or carbonates such as dawsonite by proper calcination, or it may be prepared by precipitation of aluminum hydroxide from solutions of aluminum sulfate, nitrate, chloride, or different other salts, and dehydration of the precipitate of aluminum hydroxide by heat. Usually it is desirable and advantageous to further treat it with air or other gases, or by other means to activate it prior to use.

Two hydrated oxides of aluminum occur in nature, to wit, bauxite having the formula AlzOa.2I-I2O and diaspore having the formula AI2O:.H2O. Of these two minerals only the corresponding oxide from the bauxite is suitable for the manufacture of the present type of catalysts and this material has generally given the best results of any of the supporting materials which have been tried. The mineral dawsonite having the formula NazAl(CO3)a.2Al(OH) is another mineral which maybe used as a source of aluminum oxide, the calcination of this mineral giving an alkalized aluminum oxide which is apparently more effective as a support in that the catalyst is more readily regenerated after a period of service. ,Alumina in the form of powdered corundum is not suitable as a support.

It is best practice in the final steps of preparing aluminum o xide as a base catalyst to ignite 'it for some time at temperatures within the approximate range of from 600-700 C. This does not correspond to complete dehydration of the oxide but gives a catalytic material of good strength and. porosity so that it is able to resist for'a long period of time the deteriorating effects of the service and reactivation periods to which it is subjected.

The element chromium has several .oxides, the four best known being CrO, ClzOa, CI'SO-i, and Cl'Os. The sesquioxide CrzOe is readily produced by heating salts of chromium or the trioxide in hydrogen or hydrocarbon vapors at temperatures above 300 C. The dioxide has been considered to be an equimolecular mixture of the trioxide and the sesquioxide. The oxides are readily developed on the surfaces and pores of alumina granules by utilizing primary solutions of chromic acid H2CrO4 or chromium nitrate Cr(NOa)a. The ignition of the chromic acid, the

nitrate or a precipitated trihydroxide produces the sesquioxide to furnish one of the catalytic oxide M00: and sesquioxide M0203. Since the reduction of the'triox'ide by hydrogen begins at 300 C. (572 F.) and the-reduction, is rapid at 450 C. (842 F.) the effective catalytic material is principally the sesquioxide. The trioxide may be added to the active alumina carrier from a solution in aqueous ammonia or from-a solution of ammonium molybdate which are added in amounts just requisite to wet the carrier granules uniformly and the mass is then dried and ignited.

The following example of the preparation of a catalyst comprised within the scope of the invention and its use in dehydrocyclization reactions is given to show the advantages and practicality of the invention although not with the intent of imposing undue limitations thereon.

A granular activated alumina comprising particles of approximately 4:12 mesh was given successive additions of chromium sesquioxide, molybdenum sesquioxide, and a further amount of the chromium compound by first adding a solution of chromium nitrate of a concentrationand in an amount sufllcient to leave a residue of about 2% CmOl by weight after reduction of the primary oxides. After drying, a solution of ammonium molybdate was then added to produce ultimately a deposit of M020: after which the primary step of adding the chromiumnitrate solution was repeated so that ultimately after drying and reducing the composite catalyst had the following approximate composition:

Per cent A1203- 92 CrzOc 4 M0203 k 4 The composite material was subjected to reduction in a current of hydrogen at a temperature of approximately 500 C. to reduce allthe primarily deposited oxides to the sesquioxide,

For comparative purposes and to show the ad- Once-through Catalyst yield of toluene Percent 92% A\zO3-3% Ono, 92% AhOr-8% M0101 22 92% A] 20a-4% Choc-4% Minor 31 It will be seen from the above data that the activity of the catalyst comprising 92% alumina and supporting 4% each of chromium and molybdenum sesquioxides was approximately 50% greater than that of the catalysts comprising respectively 92% aluminum oxide and 8% chromium sesquioxide or molybdenum sesquioxide. It was further observed that the rate of carbon formation due to side reactions was much tion and numerical data although neither section is intendedto. be unduly limiting thereon. We-claimas, our invention: i. A, process for the production of aromatic hydrocarbons .from aliphatic hydrocarbons of from six to twelve carbon atoms, which comprises dehydrogenating and cyclizing the allphatic-hydrocarbon bysubjection to a temperature of the order of 400 to'700 C. for aperlod of time not exceeding 10 seconds in the presence of a catalyst comprising molybdenumoxide and chromium oxide. V

2. A process for the production of aromatic hydrocarbons from aliphatic hydrocarbons of from 'six to twelve carbon atoms, which comprises dehydrogenating and cyclizing the allquioxide.

phatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period of time not exceeding 10 seconds in the presence of a catalyst comprising a major proportion of aluminum oxide and minor proportions of molybdenum oxide and chromium oxide.

3. A process for the production of aromatic hydrocarbons fromaliphatic hydrocarbons 01' from six to twelve carbon atoms, which comprises dehydrogenating and cyclizing the aliphatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period of time of about 1 to 10 seconds in the presence of a catalyst comprising molybdenum sesquioxide and chromium sesquioxide.

4; A process for the production of aromatic --hydrocarbons from aliphatic hydrocarbons of vfrom six to twelve carbon atoms, which comactivated aluminum oxide and minor proportions of molybdenum sesquioxide and chromium ses- ARISTID v. GROSSE. JACQUE c. MORREIL. 

